Biaryl refers to compounds in which two aryl groups are joined to one another via a single bond. The simplest biaryl is biphenyl.

Scheme 1: Biphenyl as the Simplest Biaryl
Phenol-arene derivatives are compounds in which one aryl unit bears an OH group (“phenol”) and the other aryl unit (“arene”) bears no free OH groups. Alkoxy groups, i.e. protected OH groups, on the arene ring are permitted however.
Biphenols and biaryls serve as synthesis units for catalytically active substances and are therefore of industrial interest. Biaryls are important units for liquid crystals, organic devices, dyes, ligands for metal catalysts, and find uses even in medical areas, since these structures are ubiquitous in biologically active, naturally occurring products (cf. a) R. Noyori, Chem. Soc. Rev. 1989, 18, 187; b) I. Cepanec, Synthesis of Biaryls, Elsevier, New York, 2004). The 2,2′-biphenols in particular can be used for this purpose (cf. WO 2005/042547). These are employed particularly as ligand components for catalysts. In this case, the biphenol can be used, for example, as ligand unit in the enantioselective catalysis (cf. Y. Chen, S. Yekta, A. K. Yudin, Chem. Rev. 2003, 103, 3155-3211; J. M. Brunel Chem. Rev. 2005, 105, 857-898; S. Kobayashi, Y. Mori, J. S. Fossey, Chem. Rev. 2011, 11, 2626-2704).
Direct coupling of unprotected phenol derivatives under conventional organic conditions has been possible only in a few examples to date. For this purpose, usually superstoichiometric amounts of inorganic oxidizing agents such as AlCl3, FeCl3, MnO2, or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, which is organic, have been used (cf. G. Sartori, R. Maggi, F. Bigi, M. Grandi, J. Org. Chem. 1993, 58, 7271).
Alternatively, such coupling reactions are conducted in a multistage sequence. In this case, leaving functionalities and often toxic, complicated transition metal catalysts based on palladium, for example, are used (cf. L. Ackermann, Modern Arylation Methods, Wiley-VCH, Weinheim, 2009, X. Chen, K. M. Engle, D.-H. Wang, J.-Q. Yu, Angew. Chem. Int. Ed. 2009, 48, 5094-511, I. V. Seregin, V. Gevorgyan, Chem. Soc. Rev. 2007, 36, 1173-1193, G. Dyker, Handbook of C—H Transformations, Wiley-VCH, Weinheim, 2005).
A great disadvantage of the abovementioned methods for phenol coupling is the need to operate in anhydrous solvents with exclusion of air. Superstoichiometric amounts of the relevant oxidizing agent are frequently required. The scarcity of raw materials (e.g. boron and bromine) leads to rising prices which leads to uneconomical processes. Multi-stage syntheses require the use of different solvents and multiple purification up to attainment of the desired product.
One possible way of synthesizing these biphenols is by means of electrochemical processes. In this context, carbon electrodes such as graphite, glassy carbon, boron-doped diamond (BDD) or noble metals such as platinum were used; cf. WO2010139687A1 and WO2010023258A1. A disadvantage of these electrochemical methods is the cost of some of the apparatus, which has to be manufactured specially. Moreover, scale-up to the ton scale, as is typically required in industry, is sometimes very complex and in some cases even impossible. In particular, scale-up of the electrode materials (BDD) is currently still not possible.
Furthermore, the preparation by electrochemical methods requires the use of sometimes costly conductive salts, the reusability of which cannot be ensured. The technical complexity of electrochemical reactions is also immense. Therefore, the preparation of large electrode surfaces of BDD is only possible to a limited extent and is linked with high costs. Even small defects in BDD surfaces moreover can lead to a complete destruction of the electrodes.
The coupling of arenes and phenols to give the corresponding phenol-arene derivatives is a great challenge just like the coupling of arenes or phenols to give the corresponding biaryl or biphenol derivatives, respectively, since these reactions are often neither regioselective nor chemoselective.